Insurer American International Group Inc., succumbing to relentless investor pressure that drove its shares down 31 percent on Friday – and another 53 percent on Monday – is pulling together a survival plan that includes selling off some of its most valuable assets, raising more capital and going to the Federal Reserve for help, people familiar with the situation said.

The measures are aimed at staving off a downgrade by major credit-rating firms. AIG executives worried that such an action would set off a chain reaction that could be fatal to the firm. The insurer, which has already raised $20 billion in fresh capital so far this year, was seeking to raise an additional $40 billion to avoid a downgrade.

During a weekend scramble to shore up its finances, AIG turned down a capital infusion from a group of private-equity firms led by J.C. Flowers & Co. because an option tied to the offer would have effectively given them control of the company, an 89-year-old giant that does business in nearly every corner of the world.

The proposed option would have allowed the firms to acquire AIG for $8 billion under certain conditions. That price is just one-fourth of AIG's current market value.